1. Field of the Invention
This invention relates generally to television compatible optical scanners and more particularly to mechanical raster scanners as used in thermal imaging systems, laser display systems, laser radar systems, general purpose optical imaging and television video systems.
2. Prior Art
The invention addresses several continuing problems in electro-optics technology wih special emphasis on those experienced in high performance, high resolution thermal imaging systems (FLIR - Forward Looking InfraRed). The typical fast framing thermal imaging system or Forward Looking InfraRed collects, spectrally, filters and focuses the infrared radiation within the field of view onto an array of detectors. The detectors convert the optical signals into electrical signals which are amplified and processed for display on a video monitor. The image for a high performance system is typically provided on a television-type monitor operating at television frame rates. This is all accomplished in real time. While the special needs of thermal imaging are particularly relevant, the invention is applicable and cost effective for use in the noted additional fields as well.
The technology of using cryogenically cooled infrared detectors sensitive in the long wavelength infrared region between 3.5 and 12 microns to make real time (instantaneous rather than film recorded) images dates back to the early 1960's. In terms of image quality, sensitivity, spatial resolving power, the best devices produced imagery roughly comparable to blank and white television. The design and preformance requirements that dictated the early technology were determined by military tasks which today are essentially unchanged. Infrared detectors with near theoretical performance capability were available then as now so that implementing a given system performance requirement meant simply using a sufficient number of such detectors. Arrays of up to several hundred detectors and mechanical scanners formed the basis of the technology then as now.
All of the high performance thermal imaging systems use one of two mechanical raster scan concepts in combination with an array of cryogenically cooled detectors. One of these concepts employs a large array of up to 180 detectors oriented perpendicular to line scan dimension. The signals from the detectors are amplified and directly displayed by synchronously scanned light-emitting multiplexed to drive a cathode ray tube. This concept is illustrated in U.S. Pat. No. 3,760,181 issued to Daly et al.
A second approach is disclosed in Laakmann, U.S. Pat. No. 3,723,642. In this implementation, a short array of detectors, such as ten to thirty detectors, is scanned two-dimensionally across the image. The detectors are oriented parallel to the line scan dimension of the television raster to be generated. The signals from the detectors are summed appropriately in a delay line and processed to provide the image. Since each detector sees a perfect cold stop, this implementation provides thermal sensitivity equal to the less efficient Daly et al implementation.
An optical scanner which can accomplish two-dimensional scanning at commercial television rates is described in Wheeler, U.S. Pat. No. 3,764,192. The Wheeler patent provides for the generation of a non-astigmatic pupil by generating an apparent pupil onto a facet mirror. Any telescope or relay lens beyond the framing mirror of Wheeler sees a field angle due to both elevation and azimuth scans.
The progress in the technology has been substantial. While the early devices were put together in a research laboratory atmosphere with little concern about cost, modern devices are being built at costs of a few percent (inflation adjusted) of the early sensors. Essentially, progress in the field has been made on the one hand by better materials technology and more cost effective systems technology and on the other hand by conventional manufacturing implementation, standardization and volume manufacturing.
A key element in the evolution of the technology was the development of tri-metal detectors such as Mercury-Cadmium Telluride which permit the use of liquid nitrogen (or argon) cooled detectors at 77 to 90 degrees K rather than the earlier doped germanium detectors that required cooling to 20 to 30 degrees K. The increased operating temperature range of HgCdTe detectors was very significant in simplifying associated refrigerators and also in allowing the use of cryostats and open cycle air, nitrogen or argon gas cooling. However, another significant difference was that the electrical bandwidth of the detector is more than two orders of magnitude larger than the doped germanium detector.
The increased electrical bandwidth of HgCdTe detectors over prior detectors made possible the system disclosed in U.S. Pat. No. 3,723,642. That system implementation capitalized on the bandwidth of the material and made possible thermal imaging systems that produced a direct conventional television video output at the U.S. 525 line rate or the European 625 line rate, at 30 or 25 frames per second, respectively. The detector bandwidth was a direct match to the 4 Mhz televison rate. Since all system detectors see all points of the image serially, these systems are commonly called "serial scan systems".
In contrast with the teaching of U.S. Pat. No. 3,723,642, prior art thermal imaging systems either used special displays having non-standard synchronization rates or used an optical or CRT-type scan converter. The advantages of direct television compatibility without costly and degrading scan converters led to the adoption of this general technology for low cost, high performance thermal imaging systems.
The teaching of Pat. Nos. 3,723,642 and 3,760,181 and a number of later related disclosures of what are commonly called "series-parallel" systems, form the base of thermal imaging sensors. The Daly, et al system implementation (3,760,181), despite its cost and size disadvantages, is the basis for the major share of plant investment and installed sensors for the United States and NATO military market. The most commonly accepted reason for this is that both developments took place in a time frame in which the military was virtually the entire market and it was felt that the need the standardization was most urgent. Because the serial scan technology disclosed in U.S. Pat. No. 3,723,642 had several elements which were not well understood or proven reliable, it was considered to be too risky for a perceived twenty year commitment of U.S. and NATO resources in the amount of many billions of dollars. On the other hand, the Daly, et al implementation was evolutionary and was indeed of low technical risk. A management decision was made by the military to satisfy all applications with the latter technology and initiate heavy funding for factory implementation.
There are many other patents and technologies which relate to commercial, non-military applications of television compatible optical scanners. These systems are generally not concerned with high resolution, long-range reconnaissance but analyze objects at close range or with very wide fields of view. An example is U.S. Pat. No. 4,349,843 to Laakmann, et al. It describes the first direct television rate scanner conceived specifically for low cost infrared radiometry. The difference between a high performance device such as described under serial and parallel Forward Looking InfraRed (FLIR) system sensors and this low cost device is not primarily in the quality of the image or in the thermal sensitivity but in the D.theta. product, where D is the diameter of the optics and .theta. is the angular extent of each detector in the system, as explained at column 1 beginning at line 60 of the specification of Patent No. 4,349,843.
These parameters are very significant in terms of device manufacturing cost. For example, if one considers the manufacturing cost of the above low cost scanner first, for commercial quantities of about 1,000/year, a figure of about $8,000.00 per system in 1985 dollars should be considered typical. On the other hand, a television compatible Forward Looking InfraRed (FLIR) systems of the parallel variety in the same quantities typically cost the military about $100,000.00 each. The average serial scan Forward Looking InfraRed (FLIR) system has a production cost in similar quantities of about $25,000.00 each. This comparison assumes devices of similar bandwidth (image quality) and thermal sensitivity. It does not include telescopes or other accessories.
The primary element in the cost difference between the low cost device and the serially scanning Forward Looking InfraRed (FLIR) system is the detector and delay line processor as all other elements are essentially the same. In the case of the Daly et al implementation, it is the complicated 180 element detector and high level processor (including the scan converter). The total system cost differences are not due to differences in scanner cost. In fact, the requirement for scanning large D.theta. products in non-military applications tends to raise the cost of scanning.
Aside from costs there are other technical considerations in principal favoring a serially scanning system. Chief among these considerations is the freedom from image artifacts and the ability to do image processing of video over a large dynamic range and before sampling. A blackbody referenced true d.c. coupled video is routinely implemented. The inability to provide a true blackbody referenced image in the parallel scanning Forward Looking InfraRed (FLIR) system limits the safe application of these devices as aircraft night navigation and landing aids. The a.c. coupled nature of the processing channels causes blackout (or white out) of the display during banking maneuvers due to the extreme thermal contrast between sky and ground.
U.S. Pat. No. 3,764,192 and U.S. Pat. No. 3,723,642 disclose inventions which are designated for specific tasks in implementing a direct television scan. Both have limitations when applied to the general case of scanning in military performace Forward Looking InfraRed (FLIR) systems. One of the limitations of both is that they are limited in the number of detector elements that they can scan in a television compatible system. A second limitation is that the ball bearings are operated in a vacuum to facilitate drag reduction for the relatively large polygon mirror and to permit operation at low power. This causes evaporation of lubricant and reduces bearing life.
Another important limitation is that the detector array has to be tailored to each scanner design, rather than using "off the self" arrays for which tooling and proven implementation exists.